Glossary of Terms for Infrared Thermal Imaging

A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z

 

A

ABSORBANCE (A)   Ratio of radiant energy absorbed by a body to the corresponding absorption of a blackbody at the same temperature. Absorbance equals emittance on bodies whose temperature is not changing. (A = 1 - R - T, where R is the reflectance and T is the transmittance.).

B

BAND PASS FILTER     An optical or detector filter which permits the passage of a narrow band of the total Spectrum. It excludes or is opaque to all other wavelengths.

BLACKBODY     The perfect absorber of all radiant energy that strikes it. The blackbody is also a perfect emitter Therefore, both its absorbance (A) and emissivity (E) are unity. The blackbody radiates energy in predictable spectral distributions and intensities which are a function of the blackbody's absolute temperature.

BOLOMETER     Thermal detector which changes its electrical resistance as a function of the radiant energy striking it.

BRIGHTNESS PYROMETER     Uses the radiant energy on each side of a fixed wavelength of die spectrum. This band is quite narrow and usually centered at 0.65 microns in the orange-red area of the visible spectrum.

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D

(DETECTOR   A device that measures the amount of energy radiated by an object. Can be a thermal detector or a photodetector. Thermal detectors respond to radiation by changing their volume, capacitance, or generation of millivoltages; they can be thermocouples, thermopiles, pneumatic detectors, or bolometers. Their common feature is their relatively slow response. Photodetectors are Semiconductors which produce a signal in proportion to the photon flux which strikes them.

E

EMISSIVITY OR EMITTANCE (E)      The emissivity of an object is the ratio of radiant energy emitted by that object divided by the radiant energy which a blackbody would emit at that same temperature. If the emittance is the same at all wavelengths, the object is called a gray body. Some industrial materials change their emissivity with temperature and sometimes with other variables also. Emissivity always equals absorption and it also equals 1 minus the sum of reflectance and transmittance (E = A= 1 - T - R).

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G

GRAY BODY   This is an object having an emittance, of less than unity, but this emittance is constant at all wavelengths (over that part of the spectrum where the measurement takes place). This means that gray body radiation curves are identical to the ones shown in the graph, except that they are dropped down on the radiated power density scale.

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I

INFRARED     That portion of the Spectrum whose wavelength is longer than that of red light, Only the portion between 0.7 and 20 microns gives usable energy for radiation detectors.

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M

MECHANICAL EMISSIVITY ENHANCEMENT    Mechanically increasing the emissivity of a surface to near-blackbody conditions (using multiple reflection).

MICRON   .001 millimeters. 10,000 Angstrom units. A unit used to measure wavelengths of radiant energy.

N

NARROW BAND PYROMETER    A radiation pyrometer that is sensitive to only a narrow Segment of wavelengths within the total radiation spectrum. Optical pyrometers are one of the devices in this category.

O

OPTICAL PYROMETER     Also called brightness pyrometer it uses a narrow band of radiation within the visible range (0.4 to 0.7 microns) to measure temperature by color matching and other techniques. .

P

PHOTODETECTOR     Measures thermal radiation by producing an output through release of electrical changes within its body. They are small flakes of crystalline materials such as US or InSb which respond to different portions of the Spectrum, consequently showing great selectivity in the wavelengths at which they operate.

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R

RATIO PYROMETER   See two-color pyrometer

REFLECTANCE OR REFLECTIVITY (R)     The percentage of the total radiation falling on a body which is directly reflected without entry. Reflectance is zero for a blackbody, and nearly 100 percent for a highly polished surface. (R = 1 - A - T, where A is the absorbance and T is the transmissivity.)

S

SPECTRAL EMISSIVITY     The ratio of emittance at a specific wavelength or very narrow band to that of a blackbody at same temperature.

T

THERMOPILE      Measures thermal radiation by absorption to become hotter than its surroundings. It is a number of small thermocouples arranged like the spokes of a wheel with the hot junction at the hub. The thermocouples are connected in series and the output is based on the difference between the hot and cold junctions.

TOTAL EMISSIVITY     The ratio of the integrated value of all spectral emittances to that of a blackbody.

TRANSMITTANCE OR TRANSMISSIVTTY (T)    The percentage of the total radiant energy falling on a body, which passes directly through it without being absorbed. Transmittance is zero for a blackbody and nearly 100 percent for a material like glass in the visible spectrum region. (T = 1 - A - R, where A is the absorbance and R is the reflectance.)

TWO-COLOR PYROMETER     Measures temperature as a function of the radiation ratio emitted around two narrow wavelength bands. Also called ratio pyrometer.

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W

WIDE-BAND (TOTAL) PYROMETER   A radiation thermometer that measures the total power density emitted by the material of interest over a wide range of wavelengths.

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